Hybrid pneumatic engine with exhaust heat recovery
The invention relates to a four-stoke hybrid pneumatic engine of the type ensuring energy recovery, which can be used for road motor vehicles or other transportation means with a view to reducing the fuel consumption and emissions which are deemed to cause the greenhouse effect. According to the invention, the hybrid pneumatic engine has a number of cylinders which are of the conventional type and at least one modified cylinder operating an active valve actuated by a mechatronic system.
Hybrid pneumatic engine with exhaust heat recovery
Hybrid pneumatic engine with
exhaust heat recovery
Author: Liviu Giurca
Abstract: The proposed hybrid engine can use the pressured air to partially recover the
braking energy and the exhaust gas heat. Using this concept an increase in power density is
obtained at part-load (in city drive) but also at full load, concomitantly with the lowering of
the fuel consumption.
The growing concern over greenhouse gas (GHG) emissions has spawned global action
aimed at making significant future reductions. One of the identified sources of GHG
production is the automotive internal combustion engine, which accounts for roughly 30% of
all GHG emissions . As a response to this situation were proposed to the consumers the
hybrid electric vehicles having the possibility to partially recover the braking energy.
The average price paid for a conventional passenger car in emerging countries is around
12,000 € and in developed countries around 17,000 €. On the other hand, the average price of
a hybrid electric vehicle is around 23,000 €. The difference is 11,000 € in case of the
emerging countries and another 6,000 € for the developed countries.
The Pacific Northwest National Laboratory and the U-M Transportation Research Institute
made a study to evaluate the consumer interest in buying hybrid electric vehicles (HEV) . In
addition to assessing the current state of knowledge and opinions about HEVs, the survey
addressed the willingness to pay for these vehicles given different cost and fuel savings
scenarios. On average, 46 percent of those surveyed said there was some chance they would
purchase a HEV that cost $2,500 (1800 €) more than a traditional vehicle; 30 percent said
there was a chance they would buy if the HEV cost $5,000 (3600 €) more ; but just 14
percent said there was a chance if it cost an additional $10,000 (7200 €). The conclusion is
that an affordable hybrid car price musts to be similar with the conventional car.
The solution to this problem seems to be a pneumatic hybrid vehicle with reduced cylinder
displacement and high power density that may have a purchase price similar to that of current
vehicles. This type of vehicle can have similar benefits as the hybrid electric vehicles: the
braking energy recovery and the stop / start operation with the consequence of reducing fuel
consumption and CO2 emissions; Improved “fun to drive” and brio. In addition, during the
manufacturing process of the pneumatic hybrid vehicle the amount of CO2 released is less
because the weight of the vehicle is diminished.
Vehicles typically use friction brakes that throw away energy in the form of heat. In order to
compensate and reduce brake wear, drivers gear down the vehicle transmission, increasing
the engine RPM, thus allowing the engine to perform work by suctioning air. Although
effective in deceleration, this method wastes valuable energy in the form of suctioned air that
cannot be used in power mode and heating while spinning up lower gears. However, the
currently four-stroke engine cycle prevents any further practical use of this wasted energy.
Consequently, it becomes interesting to develop regenerative braking systems which act to
slow down effectively a vehicle while incorporating methods to store and recover braking
energy. The need consists in some modes of engine operation that might produce, store and
accumulate energy for later use.
Therefore, the today's vehicle market is experiencing a bifurcation from the typical four-
stroke internal combustion engine to hybrid engines. Hybrids use electric motors and battery
banks to improve fuel efficiency, adding power during acceleration and reclaiming energy
when braking and coasting. The corresponding hybrid electric vehicles come with an
increasing price and weight. On the other hand, for heavy duty vehicles (trucks and buses) the
electrification increase the weight and double the price, without clear justification.
Consequently, we need hybrid type engines that do not add weight and the cost of large,
heavy battery banks, electrical generators and motors. Furthermore, what is also needed are
hybrids that do not have conversion losses from engine power to electrical power and back
from electrical power to mechanical power. As conclusion, the main requirements are hybrids
that transfer mechanical engine energy or vehicle momentum to recoverable energy forms
which can be quickly re-introduced for engine or external uses, thus further extending the
energy produced from combustion. It is a case of the pneumatic hybrid engines (vehicles)
which achieve these requirements in a relative simple manner.
On the other hand, the pneumatic or electric hybrid vehicles continue to lose an important
energy, which is this of the exhaust gases.
In conclusion, it becomes interesting to develop a hybrid engine that offers
concomitantly the regenerative braking (for terrestrial vehicles), stop/start operation
and the exhaust gases heat recovery.
ACTUAL STAGE OF THE DEVELOPMENT
The basic idea of pneumatic hybridization is to use an internal combustion engine not only
for combustion but also for a pump and a pneumatic motor. Each cylinder of the combustion
engine is connected via a fully variable charge valve to a shared air pressure tank. In vehicle
braking phases with fuel cut-off, the engine can intake air and pump it into the pressure tank.
The pressurized air can be used to boost the conventional engine combustion mode, thereby
overcoming the turbo-lag in supercharged engines. In the case of the experiment of Swiss
Federal Institute of Technology Zurich (fig. 1 and 2) each cylinder of the combustion engine
is connected via a fully variable charge valve to a shared air pressure tank. In vehicle braking
phases with fuel cut-off, the engine can intake air and pump it into the pressure tank. The
pressurized air can be used to boost the conventional engine combustion mode, thereby
overcoming the turbo-lag in supercharged engines. Tests on the New European Drive Cycle
have proven that this technology saves between 25% and 35% fuel depending on the vehicle.
For the comparison, the engines have the same rated power and the same vehicle as a basis.
This technology use a fully actuated charge valve of a type actuated in an electro-hydraulic
manner, which increase the cost (is necessary one for each cylinder) and limit the space
necessary for the other valves. In this case the four valves per cylinder associated with direct
injection is difficult to be maintained.
Fig. 1 Fig. 2
Other solutions were experimented by Scania (fig. 3) and Brunel University (fig. 4).
Fig. 3 Fig. 4
Unfortunately these systems are very useful in city drive and but not in the highway or in
CONCEPT AND DEVELOPMENT GOALS OF A NEW CONCEPT
The following goals were pursued in the development of the hybrid engine concept:
a) Braking energy recovery in city drive and exhaust gas heat recovery in highway.
b) Reduction in friction loses due to downsizing.
c) Identical design of the components reported to the classic four stroke engine and low
d) Improved efficiency at all rpm (part loads and full load).
e) Improved power density.
f) Optimal combustion for improved consumption and performance as well as reduced
g) Design simplicity excluding the need for complex or expensive technology and precious
h) Compactness and reduced weight allowing for easy assembly and maintenance.
SYSTEM DESCRIPTION AND OPERATION
The invention relates to a four-stoke hybrid pneumatic engine of the type ensuring energy
recovery, which can be used in road motor vehicles or other transportation means with a view
to reducing the fuel consumption and emissions which are deemed to cause the greenhouse
effect. The braking energy is recovered by the Pneumatic hybrid engine in the manner
describing in the State of the art chapter by the use of active valves actuated by some
mechatronic system (electro-mechanic, mechanic, pneumatic or hydraulic).
The exhaust gas kinetic energy will be recovered in a classic manner with a turbo-compressor.
If we want to achieve the exhaust gas heat recovery, an even more complex configuration
must be added, but the base components remain the same (fig. 5). In this case one or two
cylinders of the engine can work using the compressed air and the heat of the exhaust gases
produced by the other cylinders.
According to the invention (fig. 6), the hybrid pneumatic engine (1) has a number of
cylinders (2) which are of the pneumatic conventional type and at least one modified cylinder
(3). Into the corresponding cylinder head (4), for each such a cylinder (2) or modified
cylinder (3) there are operating an active valve (9) actuated by a mechatronic system, each
active valve (9) controlling a main conduit (11) which connects each said cylinder (2) or
modified cylinder (3) to a common rail (12), said rail (12) making the connection with an
auxiliary tank (13) by means of a main pipe (14). Located in front of the modified cylinder
(2) there is also used a secondary conduit (15); staggered in relation with the main conduit
(14) and, preferably, parallel thereto, both the main conduit (14) and the secondary conduit
(15) being controlled by a rotary slide valve (16) driven into rotation by the engine cam shaft
(31), the said rotary slide valve (16) being able to describe, beside the rotary motion, an axial
positive motion controlled by an actuator (17), and the modified cylinder (3) being able to
operate both in the conventional manner and as a pneumatic engine, based on the energy of
the exhaust gases provided by the cylinders (2).
The operation of the engine is possible in five modes:
1. Conventional mode: The connection between the combustion chamber and the air
tank is obstructed. The engine works in a conventional manner.
2. Compressor mode: In the braking phase, to the end of the compression stroke, the
engine delivers compressed air in the air tank concomitantly with the slowing down
of the vehicle. The fuel is cut-off.
3. Hybrid supercharging mode: In the acceleration phase, during the beginning of the
compression stroke, the air tank delivers compressed air to the combustion chamber
restoring the volumetric efficiency. The area of the p-V diagram increases a lot and
consequently the effective efficiency is improved (fig. 7). In this phase the torque and
the power furnished by the engine raise substantially comparing with the
conventional engine. The explanation is the achievement of a variable total charge
mass in the combustion chamber while keeping the charge mass of fresh air the same.
It is thereby possible to achieve a variable effective compression ratio without
the complexity of a VCR IC engine. The expression of the efficiency is:
Where rc is the compression ratio and γ is the polytrophic exponent. That means as
the efficiency is directly proportional with the compression ratio. In the fig. 7 is
shown the p-V diagram at 100% load (in red the hybrid engine and in blue the
For part-load the improving of the efficiency is even more important (fig.
4. Pneumatic motor mode (stop and start): During the stop of the vehicle the engine is
in shut down; To start the engine, during the expansion stroke is putted in connection
the air tank with the combustion chamber and the engine works until the firing as a
5. Conventional mode combined with exhaust heat recovery mode (mainly outside of
the city but also in suburban and urban area).
Comparing with the energy balance achieved by the classic engine (fig.9) in highway, this
new concept can improve with 15 to 30% the total efficiency at medium and high constant
speed (fig. 10).
The cycle diagram specific to this phase is exposed in the figure 11.
For a middle class, gasoline car ( M=1300 kg, Cx= 0.32, S= 2.2 m²), the calculated fuel
consumptions are described in the table 1.
Cycle Fuel cons.
City 7.5 173 4.5 113 -40 %
Inter-urban 4.8 110 3.1 71 -35 %
Average 6.15 154 3.8 97 -37 %
For a typical diesel truck, the calculated fuel consumptions are described in the table 2.
Cycle Fuel cons.
City 49 1124 29.4 675 -40 %
Inter-urban 35 808 24.5 565 -30 %
For an urban bus, the calculated fuel consumption is described in the table 3.
Cycle Fuel cons.
City 35 808 21 484 -40 %
In the figure 12 is indicated the cost comparison with other solutions, considering also the
anticipated fuel economy.
AVANTAGES OF THE CONCEPT COMPARING WITH THE CONVENTIONAL
1) As engine in general:
•The engine is smaller than conventional four-stroke engine. It is employed in the vehicle for
attaining high efficiency for maximum power. The size of the engine is greatly reduced
relative to the size of the vehicle in order to minimize the effect of engine friction losses and
to maximize vehicle fuel economy.
•Having dual power, mechanic and pneumatic, it can easily act the auxiliary systems of the
engine or of the vehicle lowering the total cost. Also the pneumatic option can be used to
drive home or garage pneumatic tools without the acquisition of a separate unit.
•Identical design of the components reported to the classic four-stroke engine and
consequently very low development risks (not need to find other component suppliers).
2) As hybrid engine for terrestrial vehicles:
• Heat regeneration efficiency of 18 - 30 %
• Combining this with high specific power ( up to 1,2 kW/kg for high speed racing engines)
• Cost effective
• Uses conventional technology
• Low counter pressure in the exhaust leaves ICE function unaffected
• Startup cost for industrialization similar to a variant of an existing engine
• Novel thermodynamic cycle using air as working medium
• Compatible with all fuel types (Diesel, gasoline, ethanol, methanol, CNG, LPG, etc.)
• It is a key of the present invention to achieve a significant reduction in fuel consumption by
saving and storing the energy of vehicle motion during its deceleration, and reusing it later
throughout its subsequent acceleration and propulsion. The stop and start function and the
recover of the exhaust gas heat complete this hybrid operation, obtaining big fuel economy in
traffic jumps but also in the highway.
• Compared with an electric hybrid system the proposed solution eliminates the electric
generator, motor and battery components, which are additional to the engine. This reduces
cost, complexity, weight and bulk while providing similar function and benefits. Air, even
when compressed to high pressure, is very light, and therefore the added weight is, essentially,
limited to the weight of the reservoir. Consequently this smaller weight of the proposed
hybrid system improves substantially the vehicle fuel economy.
• The cost difference comparing with a conventional engine is quickly recuperated by the
client (in one or maximum 1.5 years, depending on legislation), which is not the case for the
hybrid electric vehicle.
1. A Study of Potential Effectiveness of Carbon Dioxide Reducing Vehicle Technologies-
2007, Ricardo Inc.